The present application relates to a liquid crystal display device of a fringe field switching (referred to below as FFS) mode. Especially, the present technology relates to a liquid crystal display device of the FFS mode in which short-circuiting between a signal line and a common electrode (also referred to below as a lower electrode) is suppressed.
Examples of liquid crystal display devices employing a lateral electric field system include a liquid crystal display device of the FFS mode in which a pair of electrodes which are a pixel electrode and a common electrode is provided only on one substrate. In the liquid crystal display device of the FFS mode, the pixel electrode and the common electrode which are used for applying an electric field to a liquid crystal layer are respectively disposed on different layers with an insulation film interposed. The liquid crystal display device of the FFS mode has a wide visual angle, high contrast capability, and higher transmittance and can be driven by low voltage, being able to perform bright display. In addition, the liquid crystal display device of the FFS mode has a large overlapping area of the pixel electrode and the common electrode when viewed from above so as to have such advantage that larger storage capacitance is additionally produced and therefore provision of a separate auxiliary capacitance electrode is not demanded.
However, in manufacturing of a liquid crystal display device, physical vapor deposition such as vacuum vapor deposition and sputtering or organometallic chemical vapor deposition by thermal decomposition has been employed as a deposition method in the related art. Therefore, in such liquid crystal display device of the FFS mode, a step is formed on a position on which a signal line and a common wiring intersect with each other due to lamination of the signal line and the common wiring, and the film thickness of a lateral surface region of the step decreases so as to more likely cause decrease of dielectric pressure. Accordingly, failures such as disconnecting and short-circuiting have sometimes appeared. In the liquid crystal display device of the FFS mode of the related art, a surface of the common wiring is covered by a lower electrode, a surface of the lower electrode is covered by a gate insulation film, a signal line is formed on a surface of the gate insulation film, and a thin film transistor TFT serving as a switching element is formed near an intersection part of a scanning line and the signal line. Therefore, in the manufacturing, after an amorphous silicon (a-Si) layer and an n+a-Si layer, for example, are formed on the whole surface of the gate insulation film, patterning is performed to form a semiconductor film for forming a TFT by photolithography. At this time, before the a-Si layer and the n+a-Si layer are patterned by the photolithography, there is a cleaning process by using pure water. In the cleaning, static electricity is generated between the pure water and the n+a-Si layer, and spark is generated between the n+a-Si layer and the common wiring due to the static electricity, so that dielectric breakdown may occur in a first insulation film which is interposed between n+a-Si layer and the lower electrode.
In formation of a metallic film on a formed fine step in a semiconductor substrate, a ratio between the film thickness in a lateral surface region of the step and the film thickness in a flat part around the step is called step coverage. When the film thickness in the flat part around the step is denoted as A and the film thickness in the lateral surface region of the step is denoted as B, the step coverage is expressed as B/A. As this value becomes larger than 1, the coverage property becomes better, while as this value becomes smaller than 1, the coverage property becomes poorer. When the film thickness of the lateral surface region of the step is smaller than the film thickness in the flat part around the step, the coverage property is poor, easily causing fine holes or cracks in the lateral surface region of the step.
If a signal line, a source electrode, a drain electrode, and the like are patterned after a source layer is formed on a surface of the first insulation film in this state, the source layer enters a broken part of a gate insulation film. Accordingly, the signal line and the lower electrode short-circuit, sometimes exhibiting a line defect. Such phenomenon occurs because the thickness of the gate insulation film covering lateral surfaces of the common wiring and the lower electrode is small. The small thickness is caused by the large step formed by the common wiring and the lower electrode and the poor step coverage of the gate insulation film covering the common wiring and the lower electrode. Causes that the step formed by the common wiring and the lower electrode becomes large are the following: the lateral surface is formed on the same position of the lateral surface of the common wiring so as to prevent the lower electrode from protruding to the scanning line side (1) because a common wiring is shifted and disposed to a side of one of the scanning lines adjacent to the region in which this common wiring is provided, so as to improve aperture ratio and display image quality, and (2) because a contact area between the common wiring and the lower electrode should be enlarged as much as possible so as to reduce contact resistance between the common wiring and the lower electrode.
For such problem, Japanese Unexamined Patent Application Publication No. 6-112333 discloses a semiconductor device that can secure sufficient dielectric breakdown strength against static electricity by doubling an insulation film. In the semiconductor device disclosed in Japanese Unexamined Patent Application Publication No. 6-112333, the insulation film has the double-layer configuration which is composed of a first insulation film which is a silicon oxide film or a silicon nitride film and a second insulation film made of a heat-resistant organic material. Further, the second insulation film is formed by depositing a liquid substance having a large viscosity, so that a corner of the second insulation film does not have a cliff shape but has a loose shape and thus the film edge part becomes loose to have a gentle slope shape (taper shape). Therefore, the film thickness of the insulation film can be increased compared to the related art, being able to secure sufficient dielectric breakdown strength against static electricity.